This invention relates to a new optical glass having good devitrification stability, good transmission, a good optical quality, refractive indices of 1.73-1.78 and Abbe numbers of 30-34.
Glasses with a simlar optical position have already been described in several publications. German AS No. 12 607 12 describes glasses with n.sub.d .ltoreq.1.86 and v.sub.d&gt; 26, which are mainly within the system SiO.sub.2 -B.sub.2 O.sub.3 -BaO-ZnO. The optical position and the good chemical stability of these glasses can only be achieved by addition of up to 25% TiO.sub.2 and of up to 7% ZrO.sub.2. This entails a deterioration of transmission in the short-wave length spectral range (around 400 nm) as well as of devitrification stability.
The PbO-ZnO glasses known from JP-OS No. 78037886 require, for crystallization-stabilization and improved chemical resistance, up to 12 wt % Al.sub.2 O.sub.3, which in turn detracts from the meltability of these glasses.
The glasses produced within the system SiO.sub.2 -BaO-La.sub.2 O.sub.3 -PbO, which are known from Japanese Pat. Nos. 75 73914 and 77 84211, require up to 25% Ta.sub.2 O.sub.5 +ZrO.sub.2 or up to 15% TiO.sub.2 and up to 20% Nb.sub.2 O.sub.5 in order to achieve the desired optical position, crystallization stablity and chemical stability. With these glasses, high costs in the first case and transmission losses as well as stronger devitrification in the second case must be expected.